Process for preparing stable photoresist compositions

ABSTRACT

A micro electromechanical system having incorporated therein a composition of matter consisting of a stable solution containing a polymer derived from a solution of a polymer containing trace metals, the derived method comprising the steps of:
         (a) providing a polymer solution containing a polymer, a first solvent and trace metals;   (b) passing said polymer solution through an acidic cation ion exchange material to remove said trace metals therefrom and thereby forming a polymer solution containing free acid radicals;   (c) precipitating said polymer from said polymer solution of step b by contacting with a second solvent wherein the polymer is substantially insoluble therein;   (d) filtering said solution and said second solvent to thereby form a solid polymer cake;   (e) contacting said cake from step d with sufficient quantities of additional said second solvent in order to remove free acid radicals therefrom;   (f) removing any residual first and second solvents from said polymer to form said stable polymer.

RELATED PATENT APPLICATIONS

This patent application is a division of pending patent application Ser.No. 12/928,331 filed Dec. 9, 2010, which is a division of Ser. No.12/800,013 filed May 6, 2010 (now U.S. Pat. No. 7,862,983) and which isa division of Ser. No. 11/601,242 filed Nov. 17, 2006 (now U.S. Pat. No.7,741,429) and which in turn is derived from Provisional patentapplication Ser. No. 60/753,309, filed Dec. 22, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processes for preparing stablephotoresist compositions.

2. Description of the Prior Art

There is a desire in the industry for higher circuit density inmicroelectronic devices that are made using lithographic techniques. Onemethod of increasing the number of components per chip is to decreasethe minimum feature size on the chip, which requires higher lithographicresolutions. The use of shorter wavelength radiation (e.g., deep UV e.g.190 to 315 nm) than the currently employed mid-UV spectral range (e.g.350 nm to 400 nm) offers the potential for higher resolution. However,with deep UV radiation, fewer photons are transferred for the sameenergy dose and higher exposure doses are required to achieve the samedesired photochemical response. Further, current lithographic tools havegreatly attenuated output in the deep UV spectral region.

In order to improve sensitivity, several acid catalyzed chemicallyamplified resist compositions have been developed such as thosedisclosed in U.S. Pat. No. 4,491,628 (Jan. 1, 1985) and Nalamasu et al,“An Overview of Resist Processing for Deep UV Lithography”, 3.Photopolymer Sci. Technol. 4, 299 (1991). The resist compositionsgenerally comprise a photosensitive acid generator and an acid sensitivepolymer. The polymer has acid sensitive side chain (pendant) groups thatare bonded to the polymer backbone and are reactive towards a proton.Upon imagewise exposure to radiation, the photoacid generator produces aproton. The resist film is heated and, the proton causes catalyticcleavage of the pendant group from the polymer backbone. The proton isnot consumed in the cleavage reaction and catalyzes additional cleavagereactions thereby chemically amplifying the photochemical response ofthe resist. The cleaved polymer is soluble in polar developers such asalcohol and aqueous base while the unexposed polymer is soluble innon-polar organic solvents such as anisole. Thus the resist can producepositive or negative images of the mask depending of the selection ofthe developer solvent. Although chemically amplified resist compositionsgenerally have suitable lithographic sensitivity, in certainapplications, their performance can be improved by (i) increasing theirthermal stability in terms of thermal decomposition and plastic flow and(ii) increasing their stability in the presence of airborne chemicalcontaminants. For example, in some semiconductor manufacturingprocesses, post image development temperatures (e.g. etching,implantation etc.) can reach 200° C. Brunsvold et al., U.S. Pat. Nos.4,939,070 (issued Jul. 3, 1990) and 4,931,379 (issued Jun. 5, 1990)disclose chemically amplified, acid sensitive resist compositions havingincreased thermal stability in the post image development stage.Brunsvold's resist compositions form a hydrogen bonding network aftercleavage of the acid sensitive side chain group to increase the thermalstability of the polymer. Brunsvold avoids hydrogen-bonding moietiesprior to the cleavage reaction because such hydrogen bonding is known tounacceptably destabilize the acid sensitive side chain. AlthoughBrunsvold resists have suitable thermal stability, they also have lowersensitivity and therefore are unsuitable in certain applications.

With respect to chemical contamination, MacDonald et al. SPIE 14662,(1991) reported that due to the catalytic nature of the imagingmechanisms, chemically amplified resist systems are sensitive towardminute amounts of airborne chemical contaminants such as basic organicsubstances. These substances degrade the resulting developed image inthe film and cause a loss of the linewidth control of the developedimage. This problem is exaggerated in a manufacturing process wherethere is an extended and variable period of time between applying thefilm to the substrate and development of the image. In order to protectthe resist from such airborne contaminants, the air surrounding thecoated film is carefully filtered to remove such substances.Alternatively, the resist film is overcoated with a protective polymerlayer. However, these are cumbersome processes.

Therefore, there was a need in the art for an acid sensitive, chemicallyamplified photoresist composition having high thermal stability andstability in the presence of airborne chemical contaminants for use insemiconductor manufacturing. Apparently, this was accomplished in theinvention outlined in U.S. Pat. No. 5,625,020 which relates to aphotosensitive resist composition comprising (i) a photosensitive acidgenerator and (ii) a polymer comprising hydroxystyrene and acrylate,methacrylate or a mixture of acrylate and methacrylate. The resist hashigh lithographic sensitivity and high thermal stability. The resistalso exhibits fair stability in the presence of airborne chemicalcontaminants. However, in the above cited prior art and in U.S. Pat. No.5,284,930 and U.S. Pat. No. 5,288,850 there is another issue ofstability in a photoresist composition in that where there are usedacidic cation exchange materials to remove trace metals from theprecursor polymer solutions, there remains free acid radicals whichcarry through the process to the resist composition and render themunstable for the end intended use. Thus, one of the objects of thepresent invention is an improved process for preparing stablephotoresist compositions.

The processes of the present invention provide methods which are fast,clean, and render the photoresist composition very stable over a periodof time.

PRIOR ART

The following references are disclosed as general backgroundinformation.

1. U.S. Pat. No. 4,898,916 discloses a process for the preparation ofpoly(vinylphenol) from poly(acetoxystyrene) by acid catalyzedtransesterification.

2. U.S. Pat. No. 5,239,015 discloses a process for preparing low opticaldensity polymers and co-polymers for photoresists and opticalapplications.

3. U.S. Pat. No. 5,625,007 discloses a process for making low opticalpolymers and co-polymers for photoresists and optical applications.

4. U.S. Pat. No. 5,625,020 discloses a process for making a photoresistcomposition containing a photosensitive acid generator and a polymercomprising the reaction product of hydroxystyrene with acrylate,methacrylate or a mixture of acrylate and methacrylate.

5. EP 0813113 A1, Barclay, discloses an aqueous transesterification todeprotect the protected polymer.

6. WO 94 14858 A discloses polymerizing hydroxystyrene without theprotecting group.

Other patents of interest are U.S. Pat. No. 4,679,843; U.S. Pat. No.4,822,862; U.S. Pat. No. 4,912,173; U.S. Pat. No. 4,962,147; U.S. Pat.No. 5,087,772; U.S. Pat. No. 5,284,930; U.S. Pat. No. 5,288,850; U.S.Pat. No. 5,304,610; U.S. Pat. No. 5,395,871; U.S. Pat. No. 5,789,522;U.S. Pat. No. 5,939,511; U.S. Pat. No. 5,945,251; U.S. Pat. No.6,414,110 B1; U.S. Pat. No. 6,787,611 B2; U.S. Pat. No. 6,759,483 B2;and U.S. Pat. No. 6,864,324 B2.

All of the references described herein are incorporated herein byreference in their entirety.

SUMMARY OF THE INVENTION

The present invention provides, in part, a method of making a stablephotoresist solution containing a polymer from a solution of a polymercontaining trace metals, said method comprising the steps of:

-   -   (a) providing a polymer solution containing a polymer, a first        solvent and trace metals;    -   (b) passing said polymer solution through an acidic cation ion        exchange material to remove said trace metals therefrom and        thereby forming a polymer solution containing free acid        radicals;

(c) precipitating said polymer from said polymer solution of step b bycontacting with a second solvent wherein the polymer is substantiallyinsoluble therein;

(d) filtering said solution and said second solvent to thereby form asolid polymer cake;

(e) contacting said cake from step d with sufficient quantities ofadditional said second solvent in order to remove free acid radicalstherefrom;

(f) adding a compatible photoresist solvent to said solid polymer cakefrom step e and mixing the two in order to dissolve said polymer in saidphotoresist solvent and thereby forming a photoresist solution; and

(g) removing any residual first and second solvents from saidphotoresist solution containing said polymer to form a stablephotoresist solution.

DETAILED DESCRIPTION OF THE INVENTION

The present invention thus provides, in part, a novel process forproducing polymers free of acid radicals and which then can be used toprovide a stable photoresist composition. While the subsequent subjectmatter is directed to certain polymers, it is to be understood that thisis exemplary and a wide range of polymers may be used to prepare polymersolutions for use in the present invention process.

Preparation of the Polymer Solution:

The following is an example of preparing a hydroxyl containing polymerof I,

either alone or in combination with an acrylate monomer having theformula II,

and/or with one or more ethylenically unsaturated copolymerizablemonomers (EUCM) selected from the group consisting of styrene,4-methylstyrene, styrene alkoxide wherein the alkyl portion is C₁-C₅straight or branch chain, maleic anhydride dialkyl maleate, dialkylfumarate and vinyl chloride, wherein alkyl is having 1 to 4 carbonatoms, comprises the following steps.Step 1—Polymerization

In this step, a substituted styrene monomer of formula III,

wherein R is either —C(O)R⁵ or —R⁵; either alone (if preparing ahomopolymer) or in combination with said monomer II, and/or one or moreof said copolymerizable monomers (EUCM) is subjected to suitablepolymerization conditions in a carboxylic alcohol solvent and in thepresence of a free radical initiator at suitable temperature for asufficient period of time to produce a polymer of correspondingcomposition.

In the above formulae I, II, and III, the following are the definitions:

-   -   i) R¹ and R² are the same or different and independently        selected from the group consisting of:        -   hydrogen;        -   fluorine, chlorine or bromine;        -   alkyl or fluoroalkyl group having the formula            C_(n)H_(x)F_(y) where n is an integer from 1 to 4,        -   x and y are integers from 0 to 2n+1, and the sum of x and y            is 2n+1; and        -   phenyl or tolyl;    -   ii) R³ is selected from the group consisting of:        -   hydrogen; and        -   methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl            t-butyl;    -   iii) R⁴ is selected from the group consisting of methyl, ethyl,        n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, t-amyl, benzyl,        cyclohexyl, 9-anthracenyl, 2-hydroxyethyl, cinnamyl, adamantly,        methyl or ethyl adamantly, isobornyl, 2-ethoxyethyl, n-heptyl,        n-hexyl, 2-hydroxypropyl, 2-ethylbutyl, 2-methoxypropyl,        2-(2-methoxyethoxyl), 2-naphthyl, 2-phenylethyl, phenyl, and the        like,    -   iv) R⁵ is C₁-C₅ alkyl, either straight or branch chain.

It is also within the scope to prepare a homopolymer of formula I fromthe monomer of formula III. As one preferred embodiment,polyhydroxystyrene (PHS) can be prepared from acetoxystyrene monomer(ASM).

The present scope thus covers (a) a homopolymer of formula I derivedfrom formula III monomer; (b) a copolymer derived from formula II andformula III monomers; (c) a copolymer derived from formula III monomersand the EUCM; and (d) a terpolymer derived from monomers of formula II,formula III and EUCM. It is also within the scope of the presentinvention to use other monomers such as norbornene monomers, fluorinemonomers and the like to form a crude polymer product to be treated bythe novel processes of the present invention.

In conjunction with Formula II (an acrylate monomer) set forth herein,some preferred acrylate monomers are (1) MAA—methyl adamantyl acrylate,(2) MAMA—methyl adamantyl methacrylate, (3) EAA—ethyl adamantylacrylate, (4) EAMA—ethyl adamantyl methacrylate, (5) ETCDA—ethyltricyclodecanyl acrylate, (6) ETCDMA—ethyl tricyclodecanyl methacrylate,(7) PAMA—propyl adamantyl methacrylate, (8) MBAMA—methoxybutyl adamantylmethacrylate; (9) MBAA—methoxybutyl adamantyl acrylate, (10)isobornylacrylate, (11) isobornylmethacrylate, (12). cyclohexylacrylate,and (13) cyclohexylmethacrylate. Other preferred acrylate monomers whichcan be used are (14) 2-methyl-2-adamantyl methacrylate; (15)2-ethyl-2-adamantyl methacrylate; (16) 3-hydroxy-1-adamantylmethacrylate; (17) 3-hydroxy-1-adamantyl acrylate; (18)2-methyl-2-adamantyl acrylate; (19) 2-ethyl-2-adamantyl acrylate; (20)2-hydroxy-1,1,2-trimethylpropyl acrylate; (21)5-oxo-4-oxatricyclo-non-2-yl acrylate; (22)2-hydroxy-1,1,2-trimethylpropyl 2-methacrylate; (23)2-methyl-1-adamantyl methacrylate; (24) 2-ethyl-1-adamantylmethacrylate; (25) 5-oxotetrahydrofuran-3-yl acrylate; (26)3-hydroxy-1-adamantyl methylacrylate; (27) 5-oxotetrahydrofuran-3-yl2-methylacrylate; (28) 5-oxo-4-oxatricyclo-non-2-yl methylacrylate.

Additional acrylates and other monomers that may be used in the presentinvention with the substituted styrene to form various copolymersinclude the following materials: Monodecyl maleate; 2-hydroxy ethylmethacrylate; isodecyl methacrylate; hydroxy propyl methacrylate;isobutyl methacrylate; lauryl methacrylate; hydroxy propyl acrylate;methyl acrylate; t-butylaminoethyl methacrylate; isocyanatoethylmethacrylate; tributyltin methacrylate; sulfoethyl methacrylate; butylvinyl ether blocked methacrylic acid; t-butyl methacrylate; 2-phenoxyethyl methacrylate; acetoaetoxyethyl methacrylate; 2-phenoxy ethylacrylate; 2-ethoxy ethoxy ethyl acrylate; beta-carboxyethyl acrylate;maleic anhydride; isobornyl methacrylate; isobornyl acrylate; methylmethacrylate; ethyl acrylate; 2-ethyl hexyl methacrylate; 2-ethyl hexylacrylate; glycidyl methacrylate; N-butyl acrylate; acrolein;2-diethylaminoethyl methacrylate; allyl methacrylate; vinyl oxazolineester of meso methacrylate; itaconic acid; acrylic acid; N-butylmethacrylate; ethyl methacrylate; hydroxy ethyl acrylate; acrylamideoil; acrylonitrile; methacrylic acid; and stearyl methacrylate.

Co-polymers having polyhydroxystyrene (PHS) and one or more of the aboveacrylate monomers are some of the materials that are made by theseprocesses.

In another embodiment in this step 1, the reaction mixture may alsocomprise an additional co-solvent. The co-solvent is selected from thegroup consisting of tetrahydrofuran, methyl ethyl ketone, acetone, and1,4-dioxane.

The carboxylic alcohol solvent is an alcohol having 1 to 4 carbon atomsand is selected from the group consisting of methanol, ethanol,isopropanol, tert-butanol, and combinations thereof. The amount ofsolvent (and/or second solvent) used is not critical and can be anyamount which accomplishes the desired end result.

The free radical initiator may be any initiator that achieves thedesired end result. The initiator may be selected from the groupconsisting of 2,2′-azobis(2,4-dimethylpentanenitrile),2,2′-azobis(2-methylpropanenitrile), 2,2′-azobis(2-methylbutanenitrile),1,1′-azobis(cyclohexanecarbonitrile), t-butyl peroxy-2-ethylhexanoate,t-butyl peroxypivalate, t-amyl peroxypivalate, diisononanoyl peroxide,decanoyl peroxide, succinic acid peroxide, di(n-propyl)peroxydicarbonate, di(sec-butyl)peroxydicarbonate, di(2-ethylhexyl)peroxydicarbonate, t-butylperoxyneodecanoate,2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane,t-amylperoxyneodecanoate, dimethyl 2,2′-azobisisobutyrate andcombinations thereof.

As a preferred embodiment, the initiator is selected from the groupconsisting of 2,2′-azobis(2,4-dimethylpentanenitrile),2,2′-azobis(2-methylpropanenitrile), 2,2′-azobis(2-methylbutanenitrile),1,1′-azobis(cyclohexanecarbonitrile), t-butyl peroxy-2-ethylhexanoate,t-butyl peroxypivalate, t-amyl peroxypivalate, and combinations thereof.

The amount of initiator is any amount that accomplishes the desired endresult. However, as a preferred embodiment, said initiator is present toabout three mole percent based upon the total moles of all of saidmonomers I, II, and said copolymerizable monomers.

The polymerization conditions are any temperature and pressure that willproduce the desired end result. In general, the temperatures are fromabout 30° C. to about 100° C., preferably from about 40° C. to about100° C., and most preferably from about 45° C. to about 90° C. Thepressure may be atmospheric, sub-atmospheric or super-atmospheric. Thepolymerization time is not critical, but generally will take place overa period of at least one minute in order to produce a polymer ofcorresponding composition.

Step 2—Purification

After the polymerization of step 1 and if one so desires, and prior tothe transesterification of step 3, the polymer from step 1 is subjectedto a purification procedure wherein the same type carboxylic alcoholicsolvent (first solvent) is used to purify the polymer via a multi-stepfractionation process. Additional first solvent is added to the polymermixture of step 1, and the resultant slurry is stirred vigorously and/orheated to boiling (about 66° C.) for several minutes, and then chilledto as low as 25° C. and allowed to stand. This permits the slurry toproduce a phase separation, and then the liquid is removed bycentrifugation, filtration, decantation or by similar means. The processis repeated at least one more time until no further purification isidentified, as for example, until a small sample of the decantedsolvent, upon evaporation to dryness shows substantially no residue.This fractionation process is generally carried out 2 to 10 times, i.e.heating, cooling, separating, and the solvent replacement.

One of the important measures of the degree of impurity of a crudepolymer produced from the polymerization of the monomers is thepolydispersity value. In general, it is desirable to have a low value,for example, less than about 3; the lower value is indicative that thepolymerization reaction was more uniform in chain length. The uniquenessof this purification step is that the desired polymer formed is, to somedegree, not soluble in the solvent and that the undesired, low molecularweight average polymers and undesired monomers are soluble in thesolvent. Thus the novel purification/fractionalization provides theremoval of these undesirable materials. In general, the polydispersityof the crude polymer is measured before, during and after thispurification/fractionalization step, with the objective of reducing thisvalue by at least about 10% of what the value of the original crudepolymer was before the purification treatment. Preferably, it isdesirable to yield a product whose polydispersity is below about 2.0. Itis to be understood that polydispersity means the ratio of weightaverage molecular weight (Mw) over the number average molecular weight(Mn) as determined by Gel Permeation Chromatography (GPC).

Step 3—Transesterification

In transesterification step, the polymer/solvent mixture from step 2 issubjected to transesterification conditions in said alcohol solvent inthe presence of a catalytic amount of a transesterification catalyst.The catalyst is such that it will not substantially react with thepolymer, or said alkyl acrylate monomer II, or with saidco-polymerizable monomers (EUCM). The catalyst is selected from thegroup consisting of (anhydrous) ammonia, lithium methoxide, lithiumethoxide, lithium isopropoxide, sodium methoxide, sodium ethoxide,sodium isopropoxide, potassium methoxide, potassium ethoxide, potassiumisopropoxide, cesium methoxide, cesium ethoxide, cesium isopropoxide,and combinations thereof, wherein the carboxylic alkoxide anion issimilar to the carboxylic alcohol solvent. It is also to be understoodthat the catalyst can be alkali metal hydroxides such as lithiumhydroxide, sodium hydroxide, potassium hydroxide, cesium hydroxide andcombinations thereof. If the monomer being used is —R where it is —R⁵,then the catalyst is a strong acid such as mineral acids such as HCL.

The amount of catalyst employed is from about 0.1 mole percent to about2 mole percent of monomer I present in the composition of said polymer.

In a preferred embodiment, the catalyst is added in step (b) as asolution in said alcohol solvent.

The temperature in step (b) is such that the transesterified by-productester formed can be continually removed from the reaction mixture toform the polymer I, II, and said copolymerizable monomer. Suchtemperatures can be from about 50° C., to about 200° C. In a preferredembodiment, the transesterification reaction is carried out at refluxtemperature of said alcohol solvent.

Step 4—Purification

This optional purification step comes before the catalyst removal step.According to this step 4, there is added to the polymer in an alcoholicsolution, a second solvent which is immiscible with said alcohol solventuntil a second layer is formed. The mixture is then stirred vigorouslyor is heated to boiling for several minutes and then allowed to standuntil cool. A discrete second layer is formed which is then removed bydecantation or similar means, and the process is repeated until nofurther purification is identified, as for example, until a small sampleof the decanted second (non-alcohol) solvent upon evaporation to drynessshows no residue. In this fashion, there are removed by-products and lowweight average molecular weight materials.

The alcoholic solution of the polymer is then subjected to distillationto remove the remaining second solvent, which was miscible in thealcohol. Most often removal of the second solvent is accomplished byazeotropic distillation; the azeotropic mixture boiling below theboiling temperature of either the alcohol or the second solvent.

Typical second solvents useful for the method of this step includehexane, heptane, octane, petroleum ether, ligroin, lower alkylhalohydrocarbons, i.e., methylene chloride, and the like.

Present Invention Process Steps:

Step A: Providing a Polymer Solution.

At this point, there has been prepared a polymer solution (as set forthabove) which contains a polymer, a first solvent, and trace metals andwhich is now the starting material for the novel process of the presentinvention.

Step B: Removal of the Trace Metals.

In view of the nature of the catalyst employed in step 3 above, it iscritical that any trace metals derived from the catalyst used be removedfrom the system. In this step, there is employed a cation-exchangeresin, preferably an acidic cation exchange resin, to accomplish thedesired end result. An acidic ion exchange resin, such as sulfonatedstyrene/divinylbenzene cation exchange resin in hydrogen-form ispreferably utilized in the present process. Suitable acidic exchangeresins are available from Rohm and Haas Company, e.g. AMBERLYST 15acidic ion exchange resin. These Amberlyst resins typically contain asmuch as 80,000 to 200,000 ppb of sodium and iron. Before being utilizedin the process of the invention, the ion exchange resin should betreated with water and then a mineral acid solution to reduce the metalion level. When removing the catalyst from the polymer solution, it isimportant that the ion exchange resin be rinsed with a solvent that isthe same as, or at least compatible with, the polymer solution solvent.The procedure in this step B may be similar to those proceduresdisclosed in U.S. Pat. No. 5,284,930 and U.S. Pat. No. 5,288,850.Generally, the cation-exchange procedure is conducted at anytemperature, pressure, and flow rate which accomplishes the desired endresult.

Step C: Precipitation of Polymer

In this step, the polymer is separated from the first solvent bycontacting the polymer solution with a second solvent in which thepolymer is substantially insoluble therein. Precipitation occurs whenthis contact process takes place. The second solvent is generally fromthe group consisting of water, hexanes, heptanes, octanes, petroleumether, ligroin, lower alkyl halohydrocarbons (such as methylenechloride), and mixtures thereof. This also includes mixed isomers of allthe alkanes.

For exemplary purposes, the polymer solution is precipitated by meteringthe 27-30% solids solution in methanol into agitated 18 MΩ, de-ionizedwater at room temperature and a 1:10 polymer solution to water ratio.The rate of stirring or agitation is such that the end result is aproduct is a filterable particulate. Metered charge rate of the polymersolution is generally 10-15 wt % of polymer solution/min into theagitated de-ionized water (e.g. 6-8 kg/min for 60 kg polymer solutioninto 600 kg DI H₂O). This charge rate allows for an optimum precipitatedparticle size, conducive with adequate filtration and washing schemes.The result of this step is a slurry mixture of precipitated polymerparticles in a water/methanol medium, in which the majority of thepolymer is insoluble (some low molecular weight fragments of the polymerand impurities may be soluble in the resultant water/methanol solution).The acidic species from the ion exchange (IEX) resin step are soluble inthe mother liquor solvent.

In conjunction with the first and second solvents, these are polymerspecific in that the first solvent is any solvent wherein the polymer issubstantially soluble therein and the second solvent is any solventwherein the polymer is substantially insoluble therein. Thus, inconjunction with the solvents mentioned herein, one solvent could beused as the first solvent for a certain polymer, but for anotherpolymer, the same solvent would not be suitable and a different solventwould be utilized.

Step D: Filtration

In this step, the polymer solution, the second solvent, and theprecipitated polymer are filtered in order to provide a substantiallysolid polymer cake. This filtration can be conducted via gravity or viause of a vacuum.

For exemplary purposes, the polymer slurry, consisting of precipitatedpolymer particles and a water/methanol solution, is filtered at roomtemperature through a 25-30 μm filter to separate/isolate the polymerparticles from the water/methanol solution. The polymer particles aretrapped on the filter and the water/methanol solution passes through,taking any soluble low molecular weight polymer fragments and impuritiesincluding acidic impurities from the acid IEX resin step with it intowaste. The result is a polymer wetcake that is wet with water/methanol.The filtration is typically performed under centrifugal force or vacuumconditions to further remove the undesired water/methanol solution fromthe polymer wetcake on the filter, down to about 40% to about 60%, forexample 50% wet.

It is also within the scope of the present invention wherein theprecipitated polymer wet cake can be resolubilized in an appropriatesolvent, precipitated in an appropriate solvent and then filtered for atleast one more time.

Step E: Washing the Polymer Cake.

In this step, the polymer cake from step D is washed at least one timewith sufficient quantities of additional and fresh second solvent inorder to remove the free acid radicals therefrom. While this stepprovides for the removal of the acid radicals therefrom, there still ispresent small quantities of said first and second solvents.

For exemplary purposes, the polymer wetcake is washed on the filter withfresh de-ionized water in order to reduce the amount of methanol in thepolymer and further remove any soluble low molecular weight fragmentsand impurities including acidic impurities from the IEX resin step. Thewashing is performed over 1-10 minutes with low agitation at roomtemperature, and atmospheric pressures. The charge amount of freshde-ionized water is about 50-67 wt % of the weight of the pre-washedwetcake, or about 30-50 wt % of the starting polymer solution prior toprecipitation. This charge amount of water washing provides for adequateremoval of the methanol from the polymer wetcake to minimizeflammability hazards and further remove any soluble low molecular weightpolymer fragments and impurities. The result is a re-slurried polymerwetcake in de-ionized water, which is then filtered/isolated as statedin the previous step to remove the wash water. Since the washing isperformed on the filter, exposure to additional impurities is minimized,and allows for timely re-isolation of the polymer wetcake. Again, thepolymer wetcake is “de-watered” to about 50 wt % wet.

Step F: Solvent Swap

In this step, the solid polymer is solvent exchanged and dissolved in anaprotic/organic solvent which is a photoresist compatible (PC) solvent.This PC solvent is at least one member selected from glycol ethers,glycol ether acetates and aliphatic esters having no hydroxyl or ketogroup. Examples of the solvent include glycol ether acetates such asethylene glycol monoethyl ether acetate and propylene glycol monomethylether acetate (PGMEA) and esters such as ethyl-3-ethoxypropionate,methyl-3-methoxypropionate, among which PGMEA is preferred. Thesesolvents may be used alone or in the form of a mixture thereof.

Further examples of the PC solvent include butyl acetate, amyl acetate,cyclohexyl acetate, 3-methoxybutyl acetate, methyl ethyl ketone, methylamyl ketone, cyclohexanone, cyclopentanone, 3-ethoxyethyl propionate,3-ethoxymethyl propionate, 3-methoxymethyl propionate, methylacetoacetate, ethyl acetoacetate, diacetone alcohol, methyl pyruvate,ethyl pyruvate, propylene glycol monomethyl ether, propylene glycolmonoethyl ether, propylene glycol monomethyl ether propionate, propyleneglycol monoethyl ether propionate, ethylene glycol monomethyl ether,ethylene glycol monoethyl ether, diethylene glycol monomethyl ether,diethylene glycol monoethyl ether, 3-methyl-3-methoxybutanol,N-methylpyrrolidone, dimethylsulfoxide, γ-butyrolactone, propyleneglycol methyl ether acetate, propylene glycol ethyl ether acetate,propylene glycol propyl ether acetate, methyl lactate, ethyl lactate,propyl lactate, and tetramethylene sulfone. Of these, the propyleneglycol alkyl ether acetates and alkyl lactates are especially preferred.The solvents may be used alone or in admixture of two or more. Anexemplary useful solvent mixture is a mixture of a propylene glycolalkyl ether acetate and an alkyl lactate. It is noted that the alkylgroups of the propylene glycol alkyl ether acetates are preferably thoseof 1 to 4 carbon atoms, for example, methyl, ethyl and propyl, withmethyl and ethyl being especially preferred. Since the propylene glycolalkyl ether acetates include 1,2- and 1,3-substituted ones, eachincludes three isomers depending on the combination of substitutedpositions, which may be used alone or in admixture. It is also notedthat the alkyl groups of the alkyl lactates are preferably those of 1 to4 carbon atoms, for example, methyl, ethyl and propyl, with methyl andethyl being especially preferred.

When the propylene glycol alkyl ether acetate is used as the PC solvent,it preferably accounts for at least 50% by weight of the entire PCsolvent. Also when the alkyl lactate is used as the PC solvent, itpreferably accounts for at least 50% by weight of the entire PC solvent.When a mixture of propylene glycol alkyl ether acetate and alkyl lactateis used as the PC solvent, that mixture preferably accounts for at least50% by weight of the entire PC solvent. In this PC solvent mixture, itis further preferred that the propylene glycol alkyl ether acetate is 60to 95% by weight and the alkyl lactate is 40 to 5% by weight. A lowerproportion of the propylene glycol alkyl ether acetate would invite aproblem of inefficient coaling whereas a higher proportion thereof wouldprovide insufficient dissolution and allow for particle and foreignmatter formation. A lower proportion of the alkyl lactate would provideinsufficient dissolution and cause the problem of many particles andforeign matter whereas a higher proportion thereof would lead to acomposition which has a too high viscosity to apply and loses storagestability.

Usually the PC solvent is used in amounts of about 300 to 2,000 parts,preferably about 400 to 1,000 parts by weight per 100 parts by weight ofthe solids in the chemically amplified positive resist composition. Theconcentration is not limited to this range as long as film formation byexisting methods is possible.

Step G: First and Second Solvent Removal.

In this step, the resultant photoresist solution containing the polymer,and residuals of the first and second solvents are subjected to adistillation process whereby the first and second solvents are removedfrom the photoresist solution thus providing a stable photoresistcomposition which has substantially no free acid radicals and no water,i.e. <5000 ppm water.

If one so desires to prepare a final photoresist composition, it isprepared without isolating the photoresist material by directly addingto the photoresist solution (prepared as described above) a photoacidgenerating (PAG) compound capable of generating an acid upon exposure toactinic radiation (photoacid generator) and if necessary a base andadditives for improvement of optical and mechanical characteristics, afilm forming property, adhesion with the substrate, etc. optionally inthe form of a solution. The viscosity of the composition is regulated byaddition of PC solvent, if necessary. The PC solvent used in preparingthe resist composition is not necessarily limited to the type of PCsolvent having been used in step F, and it is possible to use any otherPC solvent which is conventionally used in preparation of a photoresistcomposition. Further, any photo acid-generating compounds and otheradditives, which are used conventionally in chemically amplifiedresists, can also be used. The total solid content in the resistcomposition is preferably in the range of 9 to 50% by weight, morepreferably 15 to 25% by weight, relative to the solvent.

The photoacid generator is a compound capable of generating an acid uponexposure to high energy radiation. Preferred photoacid generators aresulfonium salts, iodonium salts, sulfonyldiazomethanes, andN-sulfonyloxyimides. These photoacid generators are illustrated belowwhile they may be used atone or in admixture of two or more.

Sulfonium salts are salts of sulfonium cations with sulfonates.Exemplary sulfonium cations include triphenylsulfonium,(4-tert-butoxyphenyl)diphenylsulfonium,bis(4-tert-butoxy-phenyl)phenylsulfonium,tris(4-tert-butoxyphenyl)sulfonium,(3-tert-butoxyphenyl)diphenylsulfonium,bis(3-tert-butoxyphenyl)phenylsulfonium,tris(3-tert-butoxyphenyl)sulfonium,(3,4-di-tert-butoxyphenyl)diphenylsulfonium,bis(3,4-di-tert-butoxyphenyl)phenylsulfonium,tris(3,4-di-tert-butoxyphenyl)sulfonium,diphenyl(4-thiophenoxyphenyl)sulfonium,(4-tert-butoxycarbonyl-methyloxyphenyl)diphenylsulfonium,tris(4-tert-butoxycarbonylmethyloxyphenyl)sulfonium,(4-tert-butoxyphenyl)bis(4-dimethylaminophenyl)sulfonium,tris(4-dimethylaminophenyl)sulfonium, 2-naphthyldiphenylsulfonium,dimethyl-2-naphthylsulfonium, 4-hydroxyphenyldimethylsulfonium,4-methoxyphenyl-dimethylsulfonium, trimethylsulfonium,2-oxocyclohexylcyclohexyl-methylsulfonium, trinaphthylsulfonium, andtribenzylsulfonium. Exemplary sulfonates includetrifluoromethanesulfonate, nonafluorobutanesulfonate,heptadecafluorooctanesulfonate, 2,2,2-trifluorooethanesulfonate,pentafluorobenzenesulfonate, 4-trifluoromethylbenzenesulfonate,4-fluorobenzenesulfonate, toluenesulfonate, benzenesulfonate,4,4-toluenesulfonyloxybenzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate. Sulfonium salts based oncombination of the foregoing examples are included.

Iodonium salts are salts of iodonium cations with sulfonates. Exemplaryiodinium cations are arytiodonium cations including diphenyliodinium,bis(4-tert-butylphenyl)iodonium, 4-tert-butoxyphenylphenyliodonium, and4-methoxyphenylphenylodonium. Exemplary sulfonates includetrifluoromethanesulfonate, nonafluorobutanesulfonate,heptadecafluorooctanesulfonate, 2,2,2-trifluoroethanesulfonate,pentafluorobenzenesulfonate, 4-trifluoromethylbenzenesulfonate,4-fluorobenzenesulfonate, toluenesulfonate, benzenesulfonate,4,4-toluenesulfonyloxy-benzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate. Iodonium salts based oncombination of the foregoing examples are included.

Exemplary sulfonyldiazomethane compounds include bissulfonyldiazomethanecompounds and sulfonylcarbonyldiazomethane compounds such asbis(ethlylsulfonyl)diazomethane,bis(1-methylpropylsulfonyl)diazomethane,bis(2-methylpropylsulfonyl)diazomethane,bis(1,1-dimethylethylsulfonyl)diazomethane,bis(cyclohexylsulfonyl)diazomethane,bis(perfluoroisopropylsulfonyl)diazomethane,bis(phenylsulfonyl)diazomethane,bis(4-methylphenylsulfonyl)diazomethane,bis(2,4-dimethylphenylsulfonyl)diazomethane,bis(2-naph-thylsulfonyl)diazomethane,4-methylphenylsulfonylbenzoyldiazomethane,tert-butylcarbonyl-4-methylphenylsulfonyldiazomethane2-naphthylsulfonylbenzoyldiazomethane,4-methylphenylsulfonyl-2-naphthoyldiazomethane,methylsulfonylbenzoyldiazomethane, andtert-butoxycarbonyl-4-methylphenylsulfonyldiazotmethane.

N-sulfonyloxyimide photoacid generators include combinations of imideskeletons with sulfonates. Exemplary imide skeletons are succinimide,naphthalene dicarboxylic acid imide, phthalimide, cyclohexyldicarboxylicacid imide, 5-norbornene-2,3-dicarboxylic acid imide, and7-oxabicyclo[2,2,1]-5-heptene-2,3-dicarboxylic acid imide. Exemplarysulfonates include trifluoromethanesulfonate, nonafluorobutanesulfonate,heptadecafluorooctanesulfonate, 2,2,2-trifluoroethanesulfonate,pentafluorobenzenesulfonate, 4-trifluoromethylbenzenesulfonate,4-fluorobenzenesulfonate, toluenesulfonate, benzenesulfonate,naphthalenesulfonate, camphorsulfonate, octanesulfonate,dodecylbenzenesulfonate, butanesulfonate, and methanesulfonate,

Benzoinsulfonate photoacid generators include benzoin tosylate, benzoinmesylate, and benzoin butanesulfonate.

Pyrogallol trisulfonate photoacid generators include pyrogallol,fluoroglycine, catechol, resorcinol, hydroquinone, in which all thehydroxyl groups are replaced by trifluoromethanesulfonate,nonafluorobutanesulfonate, heptadecafluorooctanesulfonate,2,2,2-trifluoroethanesulfonate, pentafluorobenzenesulfonate,4-trifluoromethylbenzenesulfonate, 4-fluorobenzenesulfonate,toluenesulfonate, benzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate.

Nitrobenzyl sulfonate photoacid generators include 2,4-dinitrobenzylsulfonate, 2-nitrobenzyl sulfonate, and 2,6-dinitrobenzyl sulfonate,with exemplary sulfonates including trifluoromethanesulfonate,nonafluorobutanesulfonate, heptadecafluorooctanesulfonate,2,2,2-trifluoroethanesulfonate, pentafluorobenzenesulfonate,4-trifluoromethylbenzenesulfonate, 4-fluorobenzenesulfonate,toluenesulfonate, benzenesulfonate, naphthalenesulfonate,camphorsulfonate, octanesulfonate, dodecylbenzenesulfonate,butanesulfonate, and methanesulfonate. Also useful are analogousnitrobenzyl sulfonate compounds in which the nitro group on the benzylside is replaced by a trifluoromethyl group.

Sulfone photoacid generators include bis(phenylsulfonyl)methane,bis(4-methylphenylsulfonyl)methane, bis(2-naphthylsulfonyl)methane,2,2-bis(phenylsulfonyl)propane, 2,2-bis(4-methylphenylsulfonyl)propane,2,2-bis(2-naphthylsulfonyl)propane,2-methyl-2-(p-toluenesulfonyl)propiophenone,2-cyclohexylcarbonyl-2-(p-toluenesulfonyl)propane, and2,4-dimethyl-2-(p-toluenesulfonyl)pentan-3-one.

Photoacid generators in the form glyoxime derivatives includebis-o-(p-toluenesulfonyl)-α-dimethylglyoxime,bis-o-(p-toluenesulfonyl)-α-diphenylglyoxime,bis-o-(p-toluenesulfonyl)-α-dicyclohexylglyoxime,bis-o-(p-toluenesulfonyl)-2,3-pentanedioneglyoxime,bis-o-(p-toluenesulfonyl)-2-methyl-3,4-pentanedioneglyoxime,bis-o-(n-butanesulfonyl)-α-dimethylglyoxime,bis-o-(n-butanesulfonyl)-α-diphenylglyoxime,bis-o-(n-butanesulfonyl)-α-dicyclohexylglyoxime,bis-o-(n-butanesulfonyl)-2,3-pentanedioneglyoxime,bis-o-(n-butanesulfonyl)-2-methyl-3,4-pentanedioneglyoxime,bis-o-(methanesulfonyl)-α-dimethylglyoxime,bis-o-(trifluoromethanesulfonyl)-α-dimethylglyoxime,bis-o-(1,1,1-trifluoroethanesulfonyl)-α-dimethylglyoxime,bis-o-(tert-butanesulfonyl)-α-dimethylglyoxime,bis-o-(perfluorooctanesulfonyl)-α-dimethylglyoxime,bis-o-(cyclohexylsulfonyl)-α-dimethylglyoxime,bis-o-(benzenesulfonyl)-α-dimethylglyoxime,bis-o-(p-fluorobenzenesulfonyl)-α-dimethylglyoxime,bis-o-(p-tert-butylbenzenesulfonyl)-α-dimethylglyoxime,bis-o-(xylenesulfonyl)-α-dimethylglyoxime, andbis-o-(camphorsulfonyl)-α-dimethylglyoxime.

Of these photoacid generators, the sulfonium salts,bissulfonyldiazomethane compounds, and N-sulfonyloxyimide compounds arepreferred.

In the chemically amplified positive resist composition, an appropriateamount of the photoacid generator is 0 to 20 parts, and especially 1 to10 parts by weight per 100 parts by weight of the solids in thecomposition. The photoacid generators may be used alone or in a mixtureof two or more. The transmittance of the resist film can be controlledby using a photoacid generator having a low transmittance at theexposure wavelength and adjusting the amount of the photoacid generatoradded.

In conjunction with step G and later additions as set forth above, it iscritical that these steps be conducted on an anhydrous basis, i.e.wherein the water level is less than about 5,000 parts per million(ppm), in order to avoid possible side reactions and provide a mechanismto provide a convenient and direct route to a resist composition withouthaving to isolate the polymer product and then carry out additionalprocessing steps.

It is to be understood that in conjunction with the purification steps,set forth above, it is within the scope of this invention to use both ofthese steps, only one of these steps or neither of these steps.

This invention is further illustrated by the following examples that areprovided for illustration purposes and in no way limits the scope of thepresent invention.

EXAMPLE 1

In a inch diameter PP column, 19 cm. Of water-wet USF A15 resin wasadded. This material was subjected to a 1 hour flush with 1000 grams ofdeionized water. The column was drained until 1 inch of deionized waterremained above the resin. 500 grams of methanol was added to the columnand drained over 2.5 hours until the liquid level reached about 1-2inches above the resin. The resin was then allowed to soak in themethanol for 12 hours. A second methanol flush was performed in the samemanner, and again allowed to soak for an additional 12 hours. After thesecond methanol soak, the resin was then flushed with an additional 500grams of methanol over 1 hour and sampled for water; the water retentionwas 0.08% as determined by KF titration.

1600 grams of a poly(4-hydroxystyrene/t-butoxystyrene) polymer (50:50mole ratio) was dissolved in methanol and the resultant polymer solutionwas passed through the ion exchange bed (prepared as set forth above)over a period of 27 hours.

400 grams of the ion exchanged polymer solution was then contacted withdeionized water (10:1 mole ratio) whereby the polymer precipitated. Thesolution and precipitated polymer were filtered on a glass-frit filterand washed twice with fresh deionized water. Each wash was about onehalf of the precipitation volume. The wet cake polymer was sampled forwater and the result was 55.6% as determined by KF titration.

222.9 grams of the polymer wet cake (44.4% solids in deionized ater) wasadded to a round bottom flask and 230.9 grams of ethyl lactate was addedto the wet cake polymer to form a 30% solids solution (based on the drypolymer, excluding water). The wet cake formed a large “dough ball” andrequired mild heating at 40 C for 3 hours and agitation for 12 hours tofully dissolve. The resultant solution was stripped under vacuum on arotovap to remove the water and raise the percentage solids to provide a38-42% range. A total of 194.3 grams of deionized water/ethyl lactatewas stripped off (however there were some losses due to an inadequatecollection of vapors), bringing the solution to 49.7% solids and a waterlevel of 0.17% based upon KF titration. The solids were adjusted byadding 48.0 grams of ethyl lactate, bringing the solution to 42.5%solids. A second addition of 12.4 grams of ethyl lactate was added tobring the solids to a range of 48% solids. A sample of this finalsolution was subjected to accelerated stability tests at 75 C for 4hours. The resultant material did not turn green in color indicatingthat the t-butoxy groups did not decompose and thus was stable.

Based upon the above description, a stable photoresist solution can beprepared from a solution of a polymer containing trace metals with amethod comprising the steps of:

(a) providing a polymer solution containing a polymer, a first solventand trace metals;

(b) passing said polymer solution through an acidic cation ion exchangematerial to remove said trace metals therefrom and thereby forming apolymer solution containing free acid radicals;

(c) precipitating said polymer from said polymer solution of step b bycontacting with a second solvent wherein the polymer is substantiallyinsoluble therein;

(d) filtering said solution and said second solvent to thereby form asolid polymer cake;

(e) contacting said cake from step d with sufficient quantities ofadditional said second solvent in order to remove free acid radicalstherefrom;

(f) adding a compatible photoresist solvent to said solid polymer cakefrom step e and mixing the two in order to dissolve said polymer in saidphotoresist solvent and thereby forming a photoresist solution; and

(g) removing any residual first and second solvents from saidphotoresist solution containing said polymer to form a stablephotoresist solution.

The polymer can be used in a micro electromechanical system.

In this illustration, the first solvent is an alcoholic solvent which isselected from the group consisting of methanol, ethanol, propanol,isoproponal, t-butanol, and mixtures thereof; the second solvent isselected from the group consisting of water, hexanes, heptanes, octanes,petroleum ether, ligroin, lower alkyl halohydrocarbons, and mixturesthereof; the first solvent is any solvent wherein the polymer issubstantially soluble therein; and the second solvent is any solventwherein the polymer is substantially insoluble therein.

What is claimed is:
 1. A micro electromechanical system havingincorporated therein a composition of matter derived from a method ofmaking a stable polymer from a solution of a polymer containing tracemetals, said method which consists of the steps of: (a) providing apolymer solution containing a polymer, a first solvent and trace metals;(b) passing said polymer solution through an acidic cation ion exchangematerial to remove said trace metals therefrom and thereby forming apolymer solution containing free acid radicals; (c) precipitating saidpolymer from said polymer solution of step b by contacting with a secondsolvent wherein the polymer is substantially insoluble therein; (d)filtering said solution and said second solvent to thereby form a solidpolymer cake; (e) contacting said cake from step d with sufficientquantities of additional said second solvent in order to remove freeacid radicals therefrom; and (f) removing any residual first and secondsolvents from said polymer to form said stable polymer.